Dog Days of Summer

The Nation’s Capital has experienced some of its most significant heat waves in August.  It was just two years ago that Washingtonians endured an 12-day heat wave that included three consecutive days of triple-digit heat (August 13 – 15, 2016).  That was the first time the District had such an occurrence since July 5 – July 8, 2012.  DC residents also experienced two days of triple digit heat in August 1997 and August 2002.

Local residents who are tired of the current hazy, hot and humid weather should take comfort in the fact that average temperatures fall during August.  The amount of daylight also falls during August.  The combination of a gradual decline in average temperatures and average amount of daylight are a sure sign of the approaching autumn.  

However, August 11 can still be a very hot day.  This date’s average high/low temperatures in Washington, D.C. are 87°/70°.  While the record high is 101° that occurred in 1900, today’s record low of 56° occurred in 1879.  Weather records in the Nation’s Capital date back to the 1870s. 

Over the last 30 years, Washingtonians have experienced wide variety of weather on today’s date.  Today’s record high-low temperature occurred in 2010 when DC’s low temperature on August 11 was a balmy 80°.  A record “high-low” temperature essentially means the coolest temperature of a given day was much warmer than average.  By comparison, today’s record low-high temperature of 69° occurred most recently in 1989.  A record “low-high” temperature means it was appreciably cooler than average on a particular day.  In this case, the high temperature was 18° below average.

Torrential rainfall has also occurred during slow-moving thunderstorms, which can sometimes lead to flooding.  That’s what happened on August 11, 2001 when 0.92” fell at National Airport and 1.6” fell at Dulles Airport.  Moreover, 3.79” of rain swamped the WUSA9 weather terrace and flash flooding occurred in parts of the DC Metro Area. 

Two years earlier on August 11, 1999, water restrictions went into effect in parts of the DC Metro Area during what had been a very hot and dry summer.  Ironically, there was a 180° shift in the weather pattern that helped end the dry conditions of that summer.  The remnants of Hurricane Dennis brought the DC Metro Area appreciable rainfall and that was followed by weakening Hurricane Floyd the following month.  More recently, Washingtonians saw record rainfall from Hurricane Irene in August 2011.

Tropical Update for the Atlantic Ocean Basin

Beryl and Chris on July 9, 2018  (Source: NOAA)

The Atlantic hurricane season, which runs from June 1 through November 30, is usually more active during the month of August.  An average hurricane season in the tropical Atlantic (including the Gulf of Mexico and Caribbean Sea) has 12 tropical storms, of which 6 become hurricanes and 3 of those become major hurricanes.  A system becomes a tropical storm and is given a name when sustained winds around the center reach 39 mph.  A tropical storm becomes a hurricane when sustained winds reach 74 mph.  A hurricane becomes “major” when reaches Category 3 status on the Saffir-Simpson Hurricane Wind Scale with sustained winds of at least 111 mph.

When hurricane season began a little over two months ago, scientists with the National Oceanic and Atmospheric Administration (NOAA) and Colorado State University (CSU) expected it to be slightly busier than average.  However, both groups update their seasonal forecasts in early August.  

The team of CSU scientists currently expect an additional nine tropical storms, of which three will become hurricanes, including one major hurricane over the remainder of season. That’s in addition to the four named storms and two hurricanes that have already developed during 2018.  Meanwhile, NOAA released its updated forecast today that calls for a total of 9 to 13 tropical storms, 4 to 7 hurricanes with up to 2 of those becoming major hurricanes.  One key factor that scientists consider when issuing a seasonal hurricane forecast for the Atlantic is sea surface temperature.  Cooler than average sea surface temperatures currently exist over much of the tropical Atlantic that is not favorable for development.  

There has also been an increased amount of wind shear across the tropics that tends to inhibit tropical development.  Several factors can contribute to increases in wind shear in the tropical Atlantic, such as the presence of El Nino.  NOAA expects ENSO (El Nino Southern Oscillation) neutral conditions to continue for the remainder of the summer and that means no El Nino currently exists.  

NOAA says the chances for an episode of El Nino developing increases to 65% during the fall months.  “El Nino” refers to the unusual warming of the central and east-central equatorial Pacific Ocean that has far-reaching repercussions in global weather.  Earlier this season, meteorologists identified several dust plumes on weather satellite that blew west from central Africa across much of the tropical Atlantic.  The presence of such thick dust plumes also contributes to making environmental conditions less favorable for tropical development. 

Conditions were much different during the very active 2017 Atlantic hurricane season when sea-surface temperatures were warmer than average and wind shear was less than it has been this season.  It’s important to remember, though, that damaging and deadly hurricanes have developed during quieter than average hurricane seasons such as Alicia in 1983 and Andrew in 1992. 

Feeling Like Summer

U.S. Supreme Court, Washington, D.C.

Washington, D.C.’s first heat wave of August officially began today when high temperatures reached the low 90s for the third consecutive day.  The last heat wave in the Nation’s Capital was July 10 – 12.  However, high temperatures over the last few weeks have largely remained below average, with a rainy weather pattern that has dominated local weather headlines.  

Since Washington, D.C. had a record-setting rainy period with 11.32” between July 17 and August 3, high temperatures only reached 90°/+ twice during this period.  By comparison, DC residents sweat through 16 days of 90°/+ during the same period in 2016 and 11 such days last year.   The combination of high temperatures in the low to mid 90s combined with high relative humidity will make the heat index (or “feels-like” temperature) feel more like 100° for the next few days.

August is traditionally the second hottest month of the year in Washington, D.C.  Over the last 25 years, there has been a wide range in the number of 90° days during the month of August with as many as 20 in 2016 to as few as one such day in August 2004.  The Nation’s Capital has averaged approximately 10 – 11 days of 90° during the month of August since 1993, including an average of three days reaching 95°/+.  With 90° temperatures on tap for much of this week, DC residents will be close to the recent August average by the upcoming weekend.

A month can be warmer than average even if daily high temperatures remain at or below average for a majority of the time.  That’s because in recent years there have been more above average overnight low temperatures.  That can largely be attributed to the increased amount of urbanization around metropolitan areas, such as Washington, D.C., as well as Philadelphia, New York City and Chicago.  Denser urban areas contribute to an enhanced “urban heat island” effect.

Despite the recent rainy period, daily low temperatures largely remained above average.  For the 17-day period from July 17 – August 3, Washington, D.C. experienced record rainfall, and daily low temperatures remained warmer than average on 11 days.  By comparison, high temperatures were above average on only four of the 17 days.  NOAA expects above average temperatures to continue in the DC Metro Area through the middle of August.

August: Final Month of Meteorological Summer

NOAA's August Temperature Outlook

Meteorologists consider June, July and August to be “the summer months.”  Not only were June and July warmer than average in the Nation’s Capital, but they were also much wetter than average.  The second half of July 2018 was the wettest on record in Washington, D.C.  This is remarkable since all of the rainfall (9.73” at National Airport) occurred without any tropical systems. 

The month of August averages 2.93” of rain in Washington, D.C., according to NOAA.  August 2018 has gotten off to a wet start with 1.28” of rain over the first two days or almost half of the monthly average.

The month of August is the second hottest of the year in Washington, D.C. with an average temperature (combining daily high and low temperatures) of 78.1°, behind only July’s average of 79.8°.  Fourteen of the last 20 Augusts have been warmer than average in the Nation’s Capital.  Average high and low temperatures fall throughout the month of August, ranging from 88°/71° on August 1 to 84°/68° on August 31. 

DC’s record high temperature for the month of August is 106° that occurred on August 6, 1918, while the coolest observed temperature is 49°.  DC has been as cool as 49° a total of three times in August; most recently on August 29, 1986. 
 

As Washington, D.C. has grown and developed in recent decades, there has been a disproportionally higher number of record high temperatures than record lows due to an increased “urban heat island effect.”  Over the last 10 years, the Nation’s Capital has tied or set a total of 13 record high temperatures (including record high-low temperatures).  That’s compared to only one record low-high temperature over the same time period.  A “low-high temperature” record often means it was a much cooler than average day, while a “high-low temperature” means it was an unusually warm day. 

NOAA expects near to slightly warmer than average temperatures this month in the Nation’s Capital.  August is also predicted to be a wetter than average month.

NOAA's Precipitation Outlook for August